The present invention relates, in general, to a process for dezincing steel scrap and, in particular, to a galvanic dezincing process in which the cathode is steel or another metal or alloy which does not have a low hydrogen overvoltage.
Zinc coated (galvanized) steel is widely used in automotive, construction, and agricultural equipment and other industries. These industries and the mills producing galvanized sheet generate a considerable quantity of fresh steel scrap, at least some of which is galvanized, which can be recycled and reused as a starting material in steel and iron-making processes. The presence of zinc in the steel scrap used in steel and iron-making processes, however, increases the cost of compliance with environmental regulations due to costs associated with dust disposal and possible pretreatment of dust as a hazardous waste, treatment of waste water for removal of zinc and collection of fumes to maintain the shop floor environment and to restrict roof-vent emissions. As a result, there is great interest in development of an economical method of removing zinc from steel scrap.
In one approach, the steel scrap is immersed in an acid such as hydrochloric acid or sulfuric acid. Iron, however, is co-dissolved with the zinc in the acid solution and the separation of the iron from the zinc has not been found to be economically feasible.
The use of caustic soda solution to dissolve zinc from galvanized steel scrap has also been proposed. An inherent advantage of this method is that iron is stable in caustic and thus, separation of iron from zinc in solution is not a significant problem. A disadvantage of this method, however, is the relatively slow rate at which zinc is removed from the galvanized surface which leads to low productivity or inadequate zinc removal.
Leeker et al. in U.S. Pat. No. 5,106,467 disclose a process for the dissolution of zinc from galvanized steel in caustic electrolyte in which the dissolution rate is accelerated by the addition of oxidizing agents such as sodium nitrate to the electrolyte. The use of nitrates, however, increases the cost of the process. In addition, the use of nitrates has been associated with the formation of cyanides and thus this approach poses a serious risk hazard.
LeRoy et al. disclose other methods for accelerating the dissolution of zinc from galvanized steel in caustic electrolyte in U.S. Pat. Nos. 5,302,260 and 5,302,261. LeRoy et al. suggest that the galvanized steel be immersed in a caustic electrolyte and electrically connected to a cathodic material which is stable in the electrolyte and which has a low hydrogen overvoltage. According to LeRoy et al., such cathodes include high-surface-area nickel-based and cobalt-based materials such as Raney nickel type and Raney Cobalt type, nickel molybdates, nickel sulfides, nickel-cobalt thiospinels and mixed sulphides, nickel aluminum alloys, and electroplated active cobalt compositions. If the scrap is clean, unpainted, or shredded, no external source of voltage is applied to the cathode material. LeRoy et al., U.S. Pat. No. 5,302,261 at col. 2, lines 37-47. If bundles of scrap are to be dezinced, however, they suggest applying an external source of voltage to the cathode to increase the rate of zinc stripping. LeRoy et al., U.S. Pat. No. 5,302,261 at col. 2, lines 47-54. The anodic dezincing of bundles or bales, however, requires large processing times, floor space and concomitant capital and electrical power costs, making this process relatively expensive. The cost of cathodic materials having a low hydrogen overvoltage also adds significantly to the cost of this approach.